It is well known that opioid drugs target three types of endogenous opioid receptors (i.e., μ, δ, and κ receptors) in biological systems. Many opiates, such as morphine, are μ opioid agonists that are often used as analgesics for the treatment of severe pain due to their activation of μ opioid receptors in the brain and central nervous system (CNS). Opioid receptors are, however, not limited to the CNS, and may be found in other tissues throughout the body, i.e., peripheral to the CNS. A number of side effects of opioid drugs may be caused by activation of these peripheral receptors. For example, administration of μ opioid agonists often results in intestinal dysfunction due to the large number of receptors in the wall of the gut (Wittert, G., Hope, P. and Pyle, D., Biochemical and Biophysical Research Communications, 1996, 218, 877-881; Bagnol, D., Mansour, A., Akil, A. and Watson, S. J., Neuroscience, 1997, 81, 579-591). Specifically, opioids are generally known to cause nausea and vomiting, as well as inhibition of normal propulsive gastrointestinal function in animals and man (Reisine, T., and Pasternak, G., Goodman & Gilman's The Pharmacological Basis of Therapeutics, Ninth Edition, 1996, 521-555), resulting in side effects such as, for example, constipation.
Recent evidence has indicated that naturally-occurring endogenous opioid compounds may also affect propulsive activity in the gastrointestinal (GI) tract. Met-enkephalin, which activates μ and δ receptors in both the brain and gut, is one of several neuropeptides found in the GI tract (Koch, T. R., Carney, J. A., Go, V. L., and Szurszewski, J. H., Digestive Diseases and Sciences, 1991, 36, 712-728). Additionally, receptor knockout techniques have shown that mice lacking μ opioid receptors may have faster GI transit times than wild-type mice, suggesting that endogenous opioid peptides may tonically inhibit GI transit in normal mice (Schuller, A. G. P., King, M., Sherwood, A. C., Pintar, J. E., and Pasternak, G. W., Society of Neuroscience Abstracts 1998, 24, 524). Studies have shown that opioid peptides and receptors located throughout the GI tract may be involved in normal regulation of intestinal motility and mucosal transport of fluids in both animals and man (Reisine, T., and Pasternak, G., Goodman & Gilman's The Pharmacological Basis of Therapeutics, Ninth Edition, 1996, 521-555). Other studies show that the sympathetic nervous system may be associated with endogenous opioids and control of intestinal motility (Bagnol, D., Herbrecht, F., Jule, Y., Jarry, T., and Cupo, A., Regul. Pept., 1993, 47, 259-273). The presence of endogenous opioid compounds associated with the GI tract suggests that an abnormal physiological level of these compounds may lead to bowel dysfunction.
It is a common problem for patients having undergone surgical procedures, especially surgery of the abdomen, to suffer from a particular bowel dysfunction called post-surgical (or post-operative) ileus. “Ileus,” as used herein, refers to the obstruction of the bowel or gut, especially the colon. See, e.g., Dorland's Illustrated Medical Dictionary, 27th ed., page 816, (W.B. Saunders Company, Philadelphia, Pa., 1988). Ileus should be distinguished from constipation, which refers to infrequency of or difficulty in feces evacuation. See, e.g., Dorland's Illustrated Medical Dictionary, 27th ed., page 375, (W. B. Saunders Company, Philadelphia,1988). Ileus may be diagnosed by the disruption of normal coordinated movements of the gut, resulting in failure of intestinal contents propulsion. See, e.g., Resnick, J., Am. J of Gastroenterology, 1997, 92, 751 and Resnick, J. Am. J of Gastroenterology, 1997, 92, 934. In some instances, particularly following surgery, including surgery of the abdomen, the bowel dysfunction may become quite severe, lasting for more than a week and affecting more than one portion of the GI tract. This condition is often referred to as post-surgical (or post-operative) paralytic ileus and most frequently occurs after laparotomy (see Livingston, E. H. and Passaro, Jr., E. D., Digestive Diseases and Sciences, 1990, 35, 121). Similarly, post-partum ileus is a common problem for women in the period following childbirth, and is thought to be caused by similar fluctuations in natural opioid levels as a result of birthing stress.
Gastrointestinal dysmotility associated with post-surgical ileus is generally most severe in the colon and typically lasts for 3 to 5 days. The administration of opioid analgesics to a patient after surgery may often contribute to bowel dysfunction, thereby delaying recovery of normal bowel function. Since virtually all patients receive opioid analgesics, such as morphine or other narcotics, for pain relief after surgery, particularly major surgery, current post-surgical pain treatment may actually slow recovery of normal bowel function, resulting in a delay in hospital discharge and increasing the cost of medical care.
Post-surgical and post-partum ileus may also occur in the absence of exogenous opioid agonists. It would be of benefit to inhibit the natural activity of endogenous opioids during and/or after periods of biological stress, such as surgery and childbirth, so that ileus and related forms of bowel dysfunction can be prevented and/or treated. Currently, therapies for ileus include functional stimulation of the intestinal tract, stool softeners, laxatives, lubricants, intravenous hydration, and nasogastric decompression. These prior art methods suffer from drawbacks, for example, as lacking specificity for post-surgical or post-partum ileus. And these prior art methods offer no means for prevention. If ileus could be prevented, hospital stays, recovery times, and medical costs would be significantly decreased, in addition to the benefit of minimizing patient discomfort. Thus, drugs that selectively act on opioid receptors in the gut would be ideal candidates for preventing and/or treating post-surgical and post-partum ileus. Of those, drugs that do not interfere with the effects of opioid analgesics in the CNS would be of special benefit in that they could be administered simultaneously for pain management with limited side effects.
Peripheral opioid antagonists that do not cross the blood-brain barrier into the CNS are known in the literature and have been tested in relation to their activity on the GI tract. In U.S. Pat. No. 5,250,542, U.S. Pat. No. 5,434,171, U.S. Pat. No. 5,159,081, and U.S. Pat. No. 5,270,328, peripherally selective piperidine-N-alkylcarboxylate opioid antagonists are described as being useful in the treatment of idiopathic constipation, irritable bowel syndrome, and opioid-induced constipation. In addition, U.S. Pat. No. 4,176,186 describes quaternary derivatives of noroxymorphone (i.e., methylnaltrexone) that are said to prevent or relieve the intestinal immobility side effect of narcotic analgesics without reducing analgesic effectiveness. U.S. Pat. No. 5,972,954 describes the use of methylnaltrexone, enteric-coated methylnaltrexone, or other quaternary derivatives of noroxymorphone for preventing and/or treating opioid- and/or nonopioid-induced side effects associated with opioid administration.
General opioid antagonists, such as naloxone and naltrexone, have also been implicated as being useful in the treatment of GI tract dysmotility. For example, U.S. Pat. No. 4,987,126 and Kreek, M. J. Schaefer, R. A., Hahn, E. F., Fishman, J. Lancet, 1983, 1, 8319, 261 disclose naloxone and other morphinan-based opioid antagonists (i.e., naloxone, naltrexone) for the treatment of idiopathic gastrointestinal dysmotility. In addition, naloxone has been shown to effectively treat non-opioid induced bowel obstruction, implying that the drug may act directly on the GI tract or in the brain (Schang, J. C., Devroede, G., Am. J Gastroenerol., 1985, 80, 6, 407). Furthermore, it has been implicated that naloxone may provide therapy for paralytic ileus (Mack, D. J. Fulton, J. D., Br. J Surg., 1989, 76, 10, 1101). However, it is well known that activity of naloxone and 7 related drugs is not limited to peripheral systems and may interfere with the analgesic effects of opioid narcotics.
Inasmuch as post-surgical and post-partum ileus, for example, are common illnesses that add to the cost of health care and as yet have no specific treatments, there is a need for a specific and effective remedy. The majority of currently known opioid antagonist therapies is not peripherally selective and has the potential for undesirable side effects resulting from penetration into the CNS. Given the estimated 21 million inpatient surgeries and 26 million outpatient surgeries each year, and an estimate of 4.7 million patients experiencing post-surgical ileus, methods involving opioid antagonists that are not only specific for peripheral systems, but also specific for the gut, are desirable for treating post-surgical and post-partum ileus.
There is still an unfulfilled need for compounds that may be used in methods to antagonize opioid receptors, particularly where undesirable symptoms or conditions are side effects of administering exogenous opioids. The present invention is directed to these, as well as other important ends.